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GPS FREQUENTLY ASKED QUESTIONS (FAQ)


What is GPS?

GPS is a satellite-based radionavigation system developed and operated by the U.S. Department of Defense (DOD). GPS permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time 24 hours a day, in all weather, anywhere in the world with a precision and accuracy far better than other radionavigation systems available today or in the foreseeable future.

GPS consists of three segments: space, control, and user.

  • The Space Segment consists of a minimum of 24 operational satellites in six circular orbits 20,200 km (10,900 NM) above the earth at an inclination angle of 55 degrees with an 11 hour 58 minute period. Although it is not a stated requirement, typically the satellites are spaced in primary orbital slots so that at any time a minimum of 6 satellites will be in view to users anywhere in the world.
  • The Control Segment consists of a master control station in Colorado Springs, with five monitor stations and three ground antennas located throughout the world. The monitor stations track all GPS satellites in view and collect ranging information from the satellite broadcasts. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes extremely precise satellite orbits. The information is then formatted into updated navigation messages for each satellite. The updated information is transmitted to each satellite via the ground antennas, which also transmit and receive satellite control and monitoring signals.
  • The User Segment consists of the receivers, processors, and antennas that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise position, velocity and time.

The GPS concept of operation is based upon satellite ranging. Users figure their position on the earth by measuring their distance from the group of satellites in space. The satellites act as precise reference points.

Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the apparent range to the satellite. Measurements collected simultaneously from four satellites are processed to solve for the three dimensions of position, velocity and time.

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How is GPS used?

GPS receivers collect signals from satellites in view. They display the user's position, velocity, and time, as needed for their marine, terrestrial, or aeronautical applications. Some display additional data, such as distance and bearing to selected waypoints or digital charts.

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Who uses GPS?

GPS is used to support land, sea, and airborne navigation, surveying, Geophysical exploration, mapping and geodesy, vehicle location systems, and a wide variety of additional applications.

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Can I use a GPS Jammer?

Jamming cell phones and GPS equipment is against the law!
In recent years, the number of websites offering “cell jammers” or similar devices designed to block communications and create a “quiet zone” in vehicles, schools, theaters, restaurants, and other places has increased substantially. While these devices are marketed under different names, such as signal blockers, GPS jammers, or text stoppers, they have the same purpose. We remind and warn consumers that it is a violation of federal law to use a cell jammer or similar devices that intentionally block, jam, or interfere with authorized radio communications such as cell phones, police radar, GPS, and Wi-Fi. Despite some marketers’ claims, consumers cannot legally use jammers within the United States, nor can retailers lawfully sell them.

Why are jammers prohibited?
Use of jamming devices can place you or other people in danger. For instance, jammers can prevent 9-1-1 and other emergency calls from getting through or interfere with law enforcement communications (ambulance, fire, police, etc). In order to protect the public and ensure access to emergency and other communications services, without interference, the FCC strictly prohibits the use, marketing, manufacture, and sale of jammers.

What happens if you use a jammer?
Operation of a jammer in the United States is illegal and may subject you to substantial monetary penalties, seizure of the unlawful equipment, and criminal sanctions including imprisonment.

Want to file a complaint or need more information?
To file a complaint alerting the FCC’s Enforcement Bureau to illegal cell, GPS, or other jamming devices, please visit www.fcc.gov/complaints or call 1-888-CALL-FCC. Additional information about jammer enforcement is available at www.fcc.gov/eb/jammerenforcement or by emailing the Enforcement Bureau at jammerinfo@fcc.gov.

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Will GPS be free in the future?

GPS will be free as stated in the Presidential Decision Document (29 March 1996) and by Congress in the 1998 Public Law (105-85)

Both state that the U.S. "will continue to provide the GPS Standard Positioning Service for peaceful civil, commercial and scientific use on a continuous, worldwide basis, free of direct user fees.

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What's the status of the GPS?

The Global Positioning System reached full Operational Capability (FOC) on July 17, 1995.

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What is the Standard Positioning Service?

GPS provides two levels of service -- a Standard Positioning Service (SPS) for general public use and an encoded Precise Positioning Service (PPS) primarily intended for use by the Department of Defense. SPS signal accuracy is intentionally degraded to protect U.S. national security interests. This process, called Selective Availability (SA), controls the availability of the system's full capabilities. The SPS accuracy specifications, given below, include the effects of SA.

SPS provides accuracy's of (for position, the accuracy with respect to geographic, or geodetic coordinates of the Earth) within:

100 meters (2 drms) horizontal 156 meters (2 Sigma) vertical 300 meters (99.99% prob.) horizontal 340 nanoseconds time (95% prob.)

SPS Coverage is continuous and worldwide, with a position dilution of precision (PDOP) of 6 or less.

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What is the status of Selective Availability (SA)?

Effective as of Midnight 01 May 2000, Selective Availability has been set to zero.

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What is GPS Rollover?

This document contains information about GPS Week 1024 Rollover.


How do I report a GPS Mapping Data error such as an incorrect address for a home or a business?

If you are having problems with information displayed by your car navigation system or handheld personal navigation device, such as location names, addresses, closed highway exits or other mapping data, it is likely that this is a mapping software problem vice a Global Positioning System (GPS) problem.

The GPS constellation of satellites does not provide any information whatsoever beyond the position, navigation and timing signals necessary for various applications and receiver types to determine location. Beyond that, it is the application software companies that build receiver capability to display maps, or to determine addresses.

Some mapping software companies produce updates only every few years so, even when buying a new vehicle with an installed navigation system, it is likely that your GPS's mapping software is already outdated by one or two years. If you do not purchase the updates when they are published, your device may be many more years out of date.

Interestingly, there are other assumptions made by these kinds of mapping applications. Perhaps you’ve found that your address does not quite line up with where your house or business is actually on the map. Consider that your address might be #60 on a street, so, lacking an actual verified position of your home or business, software algorithms assume that you would be located 6/10ths of the way up the road on the right hand side. That may not be true in that, say, perhaps a large farm or building takes up a full half of the length of a road and all other homes or businesses are actually confined to a small area down on the other end.

In any case, most businesses and homes are required to have their addresses clearly visible from the street either on the mail box and/or on the actual building. Ultimately, it is still necessary for drivers to look outside their vehicles when brought in to the vicinity of a destination by their navigation device.

TeleAtlas and NAVTEQ are the world's two big providers of mapping data. Generally, everyone else gets their mapping data from them. They both have report-a-mapping-problem links on their web sites where you can register a complaint with them.

If you wish to report an incorrect address, report or request a change to the address for a business or home, or perhaps add an address, you can submit your corrections to http://www.mapsharetool.com/external-iframe/external.jsp and http://mapreporter.navteq.com. These edits will likely be included in the next update publication of the company’s mapping data (be sure to report it in both places because they may not share). Additionally, some popular handheld navigation devices have built in an application to submit these kinds of reports and also provide a service to allow you to download frequent updates on-line.

If you determine that you are experiencing GPS issues relating to receiving the actual signals coming from the satellites, please click on this link to fill out a GPS outage report.

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